Method for connecting a die attach pad to a lead frame and product thereof

ABSTRACT

Disclosed in this specification is a semiconductor package with a die attach pad and a lead frame which are electrically and mechanically connected to one another through a conductive wire ribbon. Such a configuration reduces the package footprint and also permits different styles of die attach pads and lead frames to be interchanged, thus reducing production costs.

FIELD OF THE INVENTION

This invention relates, in one embodiment, to a semiconductor packagewherein the die attach pad is electrically and mechanically connected tothe lead frame through a conductive wire ribbon. Advantageously, such aconfiguration reduces the package footprint and also permits multipledie attach pads to be used with a single lead frame, and vice versa,thus reducing production costs. Since the lead and die attach pad areseparate, revisions and/or changes of the die attach pad traces can bedone independently, thereby lowering production costs.

BACKGROUND OF THE INVENTION

During the fabrication of a semiconductor package, a lead frame iselectrically connected to a semiconductor die. The semiconductor die ismounted on a die attach pad (DAP) and electrical connections are madebetween the leads of the lead frame and the die. A plastic molding thenencapsulates the die along with a portion of the metallic lead frame. Anexternal portion of the lead frame remains outside the molding andprovides a means for sending electrical signals to the die from outsideof the package. The external portion of the lead frame may be connectedto a printed circuit board (PCB). Typically the die attach pad is simplya segment of the lead frame, thus the die attach pad is mechanicallyconnected to the lead frame because the two components are unitary.Electrical connections are provided by gold or aluminum wires that arewirebonded between the die and the leads of the lead frame. In certain“downselt” packages, the die attach pad portion of the lead frame is“stamped” to depress the pad below the plane of the lead frame.Unfortunately, the stamping process necessitates using a package with alarge footprint, as the footprint must accommodate the sloping downsetthat is produced during the stamping process. Even a small die willrequire a large package. The downset wastes space. An alternate methodfor fabricating a package uses a separate lead frame and die attach pad.Gold wires provide the electrical connection. The pad and the frame aremechanically attached with high temperature tape or solder. The hightemperature used to activate the adhesive often precludes using dieattach pads that contain delicate components, as such components may bethermally damaged when the frame is attached.

An alternative method for attaching a die to a lead frame is desiredthat allows pads with delicate components to be connected to the leadframe.

It is also desired to provide an attachment method that may easily beadapted to connect a variety of die attach pads with a variety of leadframes.

Further, a semiconductor package is desired that minimizes the size ofthe die footprint.

SUMMARY OF THE INVENTION

The invention comprises, in one form thereof, a semiconductor packagethat uses a conductive wire ribbon to form a mechanical and electricalconnection between the die attach pad and the leads of the lead frame.

An advantage of the present invention is that various die attach padsmay be easily paired with selected lead frames. This interchangeabilityof components helps to reduce production costs and is more efficientthan prior art approaches.

A further advantage of the present invention is that down-set packagesneed not have their footprint size unduly increased.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is disclosed with reference to the accompanyingdrawings, wherein:

FIG. 1A and FIG. 1B show profile views of two packages, wherein FIG. 1Ais a profile view of a prior art downset package showing a unitary leadframe and die attach pad produced by stamping, and wherein FIG. 1B is aprofile view of a semiconductor package of the present invention;

FIG. 2A shows a top view of a die attach pad; FIG. 2B is a perspectiveview of a lead frame in FIG. 2B; and FIG. 2C is a vertical cross sectionof the die attach pad of FIG. 2A;

FIGS. 3A and 3B illustrate a perspective view of a die attach padmechanically and electrically connected to a lead frame by a wire ribbonand FIG. 3B shows a close-up view of such a connection;

FIGS. 4A and 4B depict various methods for making electrical connectionsbetween the die and the traces of the lead frame; and

FIG. 5 shows a semiconductor package after the molding compound isdisposed about the frame and pad.

Corresponding reference characters indicate corresponding partsthroughout the several views. The examples set out herein illustrateseveral embodiments of the invention but should not be construed aslimiting the scope of the invention in any manner.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Referring to FIG. 1A, an example of a prior art semiconductor package100 is shown. Package 100 includes lead frame 104 and die attach pad(DAP) 102, which are unitary. Die 110 is disposed on die attach pad 102.The assembly 100 has a footprint width 112, which is sufficiently wideto accommodate width 114 of die 110 as well as the width 106 of downstep108. Typically one or more stepwise depressions are used for downsetpackages, as this improves the stackability of the lead frames—adjacentframes are less likely to become stuck together. The details concerningstepwise downset packages are described in U.S. Pat. No. 6,246,110 toKinsman et al. (Downset Lead Frame for Semiconductor Packages).

Referring to FIG. 1B, package 120 is comprised of die attach pad (DAP)122 and lead frame 124 which are mechanically and electrically connectedto one another by conductive wire ribbon 126. The wire ribbon connectiondescribed herein is more robust than prior art solder connections. Inthe embodiment depicted in FIG. 1B, die attach pad 122 and lead frame124 are discrete components (i.e. they are not unitary) which areconnected only by conductive wire ribbon 126. Conductive wire ribbon 126provides both a mechanical connection and a electrical connectionbetween pad 122 and the leads of frame 124. Such a configurationobviates the need to use a stamped downset (see downset 108 of FIG. 1A)and thus permits the package 120 to have a reduced (relative to package100) footprint width 128 while still accommodating die 130 with width132. In the embodiment depicted in FIG. 1B, lead frame 124 has an uppersurface 124 a and a lower surface 124 b. Die attach pad 122 has an uppersurface 122 a. The depth 134 is the distance between the upper surface124 a of the frame 124 and the upper surface 122 a of the die attach pad122. An alternative view of die attach pad 122 is shown in FIG. 2A.

The wire ribbon connection is not limited to downset packages. In someembodiments, the pad is co-planar with respect to the frame, butconductive wire ribbons still form the electrical and mechanicalconnections between the two discrete components. Such an embodiment isparticularly useful when the distance between upper surface 124 a andlower surface 124 b is thick. As the frame becomes thicker, stamping thelead frame becomes increasingly difficult. The wire ribbon connectionprovides a method for making the mechanical and electrical connectionsthat obviates the need for stamping.

Another advantage of the package of FIG. 1B is that the DAP 122 and thedie 130 may be assembled in advance. Since the DAP 122 is not an initialpart of the stamped lead frame, the die 130 can be attached to the DAPbefore the lead frame is ready. The wire ribbons provide robust andversatile connections. Not only do the wire ribbons hold the DAP inplace in relation to the lead frame, but they also provide theelectrical connections to circuit traces on the DAP which are themselvesconnected to terminals of the die.

FIGS. 2A, 2B and 2C show the die attach pad 122 (FIG. 2A) and the leadframe 124 (FIG. 2B) before the wire ribbon 126 connection the twocomponents. Referring now to FIG. 2A, die attach pad 122 is comprised ofnon-conductive planar insulating region 200 and electrically conductivetraces 202 a-h. FIG. 2C is a vertical cross section of die attach pad122 that shows another view of the conductive traces 202. When a die(not shown) is placed atop die attach pad 122, electrical connectionsmay be made between terminals on the die and selected conductive traces202 a-h. The details of such electrical connections are shown in FIG. 4Aand are discussed in detail elsewhere in this specification. Insultingregion 200 is the surface of die attach pad 122. The die attach pad 122,and thus insulting region 200, can made of any suitable insultingmaterial. Examples of such insulting materials include, for example,ceramics such as alumina, aluminum nitride, or beryllium oxide.

Referring now to FIG. 2B, lead frame 124 is shown. The lead frame may beformed from any suitable conductive metal and may be coated withadditional materials to improve the physical properties (such ascorrosion resistance) of the frame. For example, the lead frame may beformed of copper and plated with a metal or combination of metals suchas nickel, palladium, zinc, and the like. Other examples of suitablematerials include nickel-iron alloys. Frame 124 has a plurality of leads204 disposed about open space 206. Tie bars 208 are disposed betweeneach of the leads 204 and will be cut away after final molding. The dieattach pad 122 is disposed adjacent to lead frame 124 such that pad 122is between the leads 204 and below space 206. The pad 122 is maintainedparallel to, but not co-planar with (e.g. directly beneath), lead frame124. In other words, the pad is in a first plane, the lead frame is in asecond plane, and the two planes are spaced from one another.Thereafter, the die attach pad 122 is wirebonded to the lead frame 124with an electrically conductive wire ribbon 126 (see FIG. 1B), thusmaking package 300 (see FIG. 3A) which has a mechanical and electricalconnection between the pad 122 and the frame 124 using the wire ribbon126.

FIG. 3A and FIG. 3B depict the package 300. Referring to FIG. 3A, dieattach pad 122 is shown disposed beneath lead frame 124. The pad 122 ismechanically and electrically connected to frame 124 by conductive wireribbons 126. FIG. 3B illustrates a close-up view of one such connection.

FIG. 3B shows wire ribbon 126 forming a mechanical and electricalconnection between frame 124 and conductive trace 202 h. Conductive wireribbon 126 may be, for example, an aluminum wire ribbon or a copper wireribbon. U.S. Pat. No. 7,216,794 provides an example of wire ribbonformation. Wire ribbons are distinguished from simple round wires bytheir greater cross-sectional area relative to a round wire. Typicalwire ribbon dimensions are, for example, 80×10 mils, 80×8 mils, 60×8mils, and 40×4 mils (cross-sectional area-width×thickness). Each of thetraces is electrically isolated from one another. For example, as shownin FIG. 2A, trace 202 h is electrically isolated from trace 202 a-g byinsulating region 200. Each of the traces 202 a-h may be placed inelectrical communication with die attach pad 122 and/or lead frame 124through a variety of means. Some of these means are depicted in FIGS. 4Aand 4B and will be discussed in detail elsewhere in this specification.

The method for forming a semiconductor package that is described hereinpermits one to select a first type of die attach pad from a plurality ofpad types and connect that first type of pad to a given lead frame.Thereafter, the manufacturer can select a second type of pad from theplurality of pads and connect that second type of pad to the same typeof lead frame. The opposite permutation is also possible, whereinvarious lead frames may be connected to a single style of die attachpad. The interchangeability of these components permits manufacturers tomore efficiently utilize their inventory.

The wire ribbon connection described in this specification also permitsone to use a die attach pad that is preassembled with delicatecomponents and attach such a pad to a lead frame without damaging thecomponents. For example, a die may be wired to the die attach pad withdelicate gold or aluminum wires. Such wires form an electricalconnection between the die and the traces within the pad. Thereafter,the traces within the pad are mechanically and electrically connected toa lead frame with the aforementioned conductive wire ribbons. Since nohigh temperatures are used in the process, the delicate wires are notdamaged during the connection step. The wires are merely one example ofestablishing an electrical connection. Other suitable connection methodswill be apparent to those skilled in the art.

FIGS. 4A and 4B illustrate various means for establishing electricalcommunication between die 130 and lead frame 410 a or 410 b. In theembodiment depicted in FIG. 4A, die attach pad 122 is comprised oftraces 402 and 404. A terminal (not shown) on the bottom surface 130 bof die 130 electrically connects die 130 to trace 402 which, in turn, isin electrical communication with wire ribbon 126 b. In this manner,electrical communication is established between die attach pad 122 andframe 410 a or 410 b. Such a wireless connection may be made when theterminal on the bottom surface 130 b is contiguous with the trace 402.One example of a die 130 that makes such a wireless connection is aflip-chip die. Alternatively or additionally, a wired connection 406 maybe used to establish an electrical connection. Package 400 contains botha wireless and wired connection. The wire 406 establishes an electricalcommunication between the top surface 130 a of die 130 and trace 404. Inone embodiment, a gold or aluminum wire is used. Such wire may beconnected using traditional wirebonding techniques. Trace 404, in turn,is mechanically and electrically connected to lead 410 a of the leadframe by conductive wire ribbon 126 a. Alternatively, as shown in FIG.4B, wire 406 may directly form an electrical connection between lead 410a and the upper surface 130 a of die 130 without an intervening trace(see package 408). In assembly 408, wire ribbon 126 b still forms both amechanical and electrical connection between die attach pad 122 and leadframe 410 b.

FIG. 5 depicts two packages 500 and 502. After the die pad 122 and leadframe 124 have been connected with wire ribbon 126, molding 504 may beformed about the assembly. The tie bars (see tie bar 208 of FIG. 2B) maybe cut and the external portions of the lead frame 124 bent into aconfiguration appropriate for the end use of the package. In package500, the die is a flip-chip die and is connected to the traces of pad122 by direct physical contact. In package 502, wires 406 connected thedie to the traces of the pad 122 prior to the attachment of the leadframe.

While the invention has been described with reference to preferredembodiments, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof to adapt to particular situations without departingfrom the scope of the invention. Therefore, it is intended that theinvention not be limited to the particular embodiments disclosed as thebest mode contemplated for carrying out this invention, but that theinvention will include all embodiments falling within the scope andspirit of the appended claims.

1. A semiconductor package comprising a lead frame with a plurality ofleads disposed in a first plane; a die attach pad with a plurality ofconductive traces isolated from each other by an insulating regions anddisposed in a second plane spaced from the first plane; a semiconductordie mounted on the die attach pad and the conductive traces such thatthe die is in electrical communication with the plurality of saidcircuit traces; and a plurality of wire ribbons extending from the leadsof the lead frame to the conductive traces on the die attach pad formechanically and electrically connecting the plurality of leads of thelead frame to the conductive traces of the die attach pad.
 2. Thesemiconductor package as recited in claim 1, wherein the wire ribboncomprises a metal selected from the group consisting of aluminum andcopper.
 3. The semiconductor package of claim 1 wherein the wire ribbonhas a first end section contacting a lead of the lead frame, a secondend section contacting a circuit trace of the die attach pad, and anintermediate section between the two end sections and disposedtransverse to the two end sections.
 4. The semiconductor package ofclaim 1, wherein the semiconductor die is a flip-chip die and the die isin electrical communication with the plurality of said circuit tracesthrough a flip-chip die lead that is contiguous with at least one traceof said plurality of circuit traces.
 5. The semiconductor package ofclaim 1, wherein the lead frame and the die attach pad are discretecomponents.
 6. The semiconductor package of claim 1, further comprisinga wire electrically connecting said die to at least one of saidplurality of conductive traces.
 7. The semiconductor package of claim 6,wherein said wire is a gold wire.
 8. The semiconductor package of claim1, further comprising a molded housing encapsulating the die attach pad,the die, the plurality of wire ribbons, and part of the lead frame suchthat a portion of the lead frame remains external to the molded housing.9. A method for making a package comprising the steps of disposing a dieattach pad with a plurality of conductive traces parallel to, but notco-planar with, a lead frame with leads that define a lead frameopening, wherein the die attach pad and lead frame are not mechanicallyconnected; wirebonding the plurality of conductive traces of the dieattach pad to the leads of the lead frame with a wire ribbon, thusmaking a connected assembly with a mechanical and electrical connectionbetween the die attach pad and the lead frame using the ribbon,encapsulating the connected assembly in a molded housing such that aportion of the lead frame remains external to the molded housing. 10.The method as recited in claim 9, wherein the die attach pad furthercomprises a semiconductor die mounted on the die attach pad such thatthe die is in electrical communication with the plurality of circuits.11. The method as recited in claim 10, wherein the semiconductor die ispresent on the die attach pad prior to the step of wirebonding the padto the lead frame.
 12. A semiconductor package comprising a lead framewith a plurality of leads; a die attach pad with a plurality ofconductive traces isolated from each other by an insulating regions, thelead frame and the die attach pad being discrete; a semiconductor diemounted on the die attach pad and the conductive traces such that thedie is in electrical communication with the plurality of said circuittraces; and a plurality of wire ribbons extending from the leads of thelead frame to the conductive traces on the die attach pad formechanically and electrically connecting the plurality of leads of thelead frame to the conductive traces of the die attach pad.
 12. Thesemiconductor package as recited in claim 12, wherein the wire ribboncomprises a metal selected from the group consisting of aluminum andcopper.
 13. The semiconductor package of claim 12, wherein the wireribbon has a first end section contacting a lead of the lead frame, asecond end section contacting a circuit trace of the die attach pad, andan intermediate section between the two end sections and disposedtransverse to the two end sections.
 14. The semiconductor package ofclaim 12, wherein the semiconductor die is a flip-chip die and the dieis in electrical communication with the plurality of said circuit tracesthrough a flip-chip die lead that is contiguous with at least one traceof said plurality of circuit traces.
 15. The semiconductor package ofclaim 12, wherein the lead frame and the die attach pad are discretecomponents.
 16. The semiconductor package of claim 12, furthercomprising a wire electrically connecting said die to at least one ofsaid plurality of conductive traces.
 17. The semiconductor package ofclaim 16, wherein said wire is a gold wire.
 18. The semiconductorpackage of claim 12, further comprising a molded housing encapsulatingthe die attach pad, the die, the plurality of wire ribbons, and part orthe lead frame such that a portion of the lead frame remains external tothe molded housing.